Require commas between moshscript arguments

[aiirvizionz]

Fixes another moshscript parser edge case: call arguments were accepted without commas even though the documented grammar requires comma-separated arguments.

Before this change, say("one" "two"); parsed as two arguments instead of failing. The parser also accepted leading and trailing commas in calls.

This PR makes argument parsing explicit:

• rejects missing commas between arguments
• rejects leading commas
• rejects trailing commas
• keeps valid comma-separated calls working

Validation:

• npm test
• node bin/moshcode.mjs run examples/alive.mosh --dry-run --max 1
• git diff --check